ZSM-12 and its conventional preparation in the presence of a tetramethylammonium or tetraethylammonium directing agent are taught by U.S. Pat. No. 3,832,449, the entire disclosure of which is incorporated herein by reference. ZSM-12 has a distinctive X-ray diffraction pattern which distingishes it from other known crystalline materials.
U.S. Pat. No. 4,391,785 discloses a method for the synthesis of ZSM-12 from a reaction mixture comprising, as a directing agent, a compound selected from the group consisting of a dimethyl pyridinium halide and a dimethyl pyrrolidinium halide.
U.S. Pat. Nos. 4,452,769 and 4,537,758 disclose methods for synthesizing ZSM-12 from a reaction mixture containing methyltriethylammonium ions as the directing agent. However, these patents are primarily directed to producing high SiO2/Al2O3 ratio forms of ZSM-12, greater than 80 in the case of the '769 patent and greater than 200 in the case of the '758 patent. Moreover, one of the stated advantages in the '769 patent of using methyltriethylammonium ions as the directing agent is the production of large crystal size materials.
Other organic directing agents that have been used to synthesize ZSM-12 include bis(dimethylpiperidinium)trimethylene ions (see U.S. Pat. No. 4,539,193), benzyltriethylammonium ions (see U.S. Pat. No. 4,552,738), dibenzyldiethylammonium ions (see EP-A-167,232), dimethyldiethylammonium ions (see U.S. Pat. No. 4,552,739), benzyltrimethylammonium ions (see U.S. Pat. No. 4,585,637), bis(N-methylpyridyl)ethylinium ions (see U.S. Pat. No. 4,5852,746), hexamethyleneimine (U.S. Pat. No. 5,021,141) and bis(methylpyrrolidinium)-diquat-n, where n=4, 5 or 6 (see U.S. Pat. No. 5,192,521).
Although influenced by variables such as the silica/alumina molar ratio of the reaction mixture, temperature and stirring, the crystal morphology of synthetic zeolites, such as ZSM-12, is mainly dominated by the choice of directing agent used in the crystallization. For example, in the case of ZSM-12, needle-shaped crystals can be produced using a benzyltrimethylammonium directing agent, rice-shaped crystals can be made in the presence of tetraethylammonium salts, and bundles of hexagonal platelets can be prepared from with a hexamethyleneimine directing agent.
The control of zeolite crystal morphology is very important from the standpoint of activity and stability enhancement. For catalytic applications, small crystal size is always the desired preference for high activity and stability because of the higher surface area, and hence the shorter diffusion path, of small crystal materials. In addition since zeolite activity is associated with the aluminum atoms in the zeolite framework, the lower the SiO2/Al2O3 ratio of the zeolite, the higher its acidity. To date, most synthesis routes for ZSM-12 have led to the production of either small crystal materials or materials with a low SiO2/Al2O3 ratio. In general, however, it has been difficult to synthesize high purity ZSM-12 which has both the attributes of small crystal size and low SiO2/Al2O3 ratio.
An object of the present invention is to provide a small crystal, high activity form of ZSM-12 which exhibits enhanced activity in the conversion of C9+ aromatic hydrocarbons to xylenes.
It is to be appreciated that, although ZSM-12 is normally synthesized as an aluminosilicate, the framework aluminum can be partially or completely replaced by other trivalent elements, such as boron, iron and/or gallium, and the framework silicon can be partially or completely replaced by other tetravalent elements such as germanium.